1. Field of the Invention
The present invention relates, in general, to a refuelable electrochemical power source, and in particular, to a refuelable electrochemical power source capable of being maintained in a substantially full or maximum electroactivity condition.
2. Related Art
One of the more promising alternatives to conventional power sources in existence today is the metal/air fuel cell. These fuel cells have tremendous potential because they are efficient, environmentally safe, and completely renewable. Metal/air fuel cells can be used for both stationary and mobile applications, and are especially suitable for use in all types of electric vehicles.
Metal/air fuel cells and batteries produce electricity by electrochemically combining metal with oxygen from the air. Zinc, Iron, Lithium, and Aluminum are some of the metals that can be used. Oxidants other than air, such as pure oxygen, bromine, or hydrogen peroxide can also be used. Zinc/air fuel cells and batteries produce electricity by the same electrochemical processes. But zinc/air fuel cells are not discarded like primary batteries. They are not slowly recharged like secondary batteries, nor are they rebuilt like "mechanically recharged" batteries. Instead, zinc/air fuel cells are conveniently refueled in minutes or seconds by adding additional zinc when necessary. Further, the zinc used to generate electricity is completely recoverable and reusable.
The zinc/air fuel cell is expected to displace lead-acid batteries where higher specific energies are required and/or rapid recharging is desired. Further, the zinc/air fuel cell is expected to displace internal combustion engines where zero emissions, quiet operation, and/or lower maintenance costs are important.
In one example embodiment, the zinc "fuel" is in the form of particles. Zinc is consumed, which releases electrons to drive a load (the anodic part of the electrochemical process). Oxygen from ambient air accepts electrons from the load (the cathodic part). The overall chemical reaction produces zinc oxide, a non-toxic white powder. When all or part of the zinc has been consumed and, hence, transformed into zinc oxide, the fuel cell can be refueled by removing the reaction product and adding fresh zinc particles and electrolyte. The zinc oxide (ZnO) product is typically reprocessed into zinc particles and oxygen in a separate, stand-alone recycling unit using electrolysis. The whole process is a closed cycle for zinc and oxygen, which can be recycled indefinitely.
In general, a zinc/air fuel cell system comprises two principal components: the fuel cell itself and a zinc recovery apparatus. The recovery apparatus is generally stationary and serves to supply the fuel cell with zinc particles, remove the zinc oxide, and convert it back into zinc metal fuel particles. A metal recovery apparatus may also be used to recover zinc, copper, or other metals from solution for any other purpose.
The benefits of zinc/air fuel cell technology over rechargeable batteries such as lead-acid batteries are numerous. These benefits include very high specific energies, high energy densities, and the de-coupling of energy and power densities. Further, these systems provide rapid on-site refueling that requires only a standard electrical supply. Still further, these systems provide longer life potentials, and the availability of a reliable and accurate measure of remaining energy at all times.
The benefits over internal combustion engines include zero emissions, quiet operation, lower maintenance costs, and higher specific energies. When replacing lead-acid batteries, zinc/air fuel cells can be used to extend the range of a vehicle or reduce the weight for increased payload capability and/or enhanced performance. The zinc/air fuel cell gives vehicle designers additional flexibility to distribute weight for optimizing vehicle dynamics.
In a prior art zinc/air fuel cell of the assignee of the present application, Metallic Power, comprise a plurality of cells electrically coupled together in a serial fashion. Before use, each cell receives a mixture of zinc particles and electrolyte. The zinc particles in each cell form an anode bed. Each cell includes an electroactive zone where zinc undergoes electrolytic dissolution. In a traditional zinc/air fuel cell or "refuelable battery," the bottom portion of a full anode bed is in the electroactive zone and immediately adjacent to a cathode. For an example of traditional zinc/air fuel cells, see U.S. Pat. No. 5,434,020 to Cooper and U.S. patent application Ser. No. 09/353,422 to Gutierrez, et al., filed Jul. 15, 1999. The zinc in the remaining upper portion is in an inactive zone called the hopper, and does not undergo electrodissolution. This zinc is regarded as fuel. Another approach of the prior art is to completely replace the zinc electrode between discharges. For example, see U.S. Pat. No. 5,441,820 to Evans, et al. and U.S. Pat. No. 5,196,275 to Goldman, et al.
In the electroactive zone, the zinc dissolves, the particles reduce in diameter and the particle bed collapses, allowing particles to fall from the hopper into the electroactive zone. When the cell is refueled, which typically occurs at the end of the discharge period when the zinc fuel level in each cell reaches the bottom of the hopper, zinc pellets are fluidized in a stream of electrolyte and pumped into a feed tube, which runs the length of the cell stack. The feed tube discharges directly into each cell until each hopper is full of zinc. The discharge and refueling phases occur sequentially.
There are several problems with this approach to cell filling. They are: 1) high electrolytic shunt currents flow between the cells via the feed tube during refueling, 2) the filling procedure requires careful balancing of flow rates and zinc volume fraction to maximize zinc transfer rates while avoiding channel blockage, 3) the relatively long filling times (ten to twenty minutes for a typical application) that result even from the best zinc transfer rates, 4) the loss of the hopper volumes within the cell stack as active, power-producing volume, 5) the need to measure the zinc level in every hopper to get a completely reliable measure of remaining fuel, and 6) the difficulty of mechanically sealing the feed tube against the cell walls sufficiently to prevent dendrite growth between filling procedures.
First, this prior art approach leads to high electrolytic shunt currents flowing through the feed tube between the cells that reduce the efficiency of the fuel cell and can lead to dendrite formation and catastrophic short-circuits between cells. Sealing each cell from the feed tube near the junction of the feed tube and each cell inlet after cell filling can reduce the shunt currents. However, the presence of zinc pellets in this area makes reliable sealing difficult. Furthermore, such a seal is a mechanical device that needs some form of motive power and the ability to open and close on the receipt of an electrical signal, adding to the cost and complexity of the fuel cell.
Second, the filling procedure for this approach is quite time-consuming and requires careful control of flow rates and zinc volume fraction, because the pellets must fill each cell in sequence and after filling, the cell inlet holes and the feeding tube must be cleared of zinc pellets. Additionally, the rate of pellet flow required to minimize cell filling time causes problems. A principal advantage of the zinc/air fuel cell is that it allows fast refueling. However, with the traditional approach, the rate of electrolyte flow and the volume fraction of pellets control the rate of filling. If the flow rate is too high, the pressure can damage the cells. If the pellet fraction is too high, the pellets can accumulate and block the passageways.
A further disadvantage with this traditional approach is that the fuel to be consumed is stored within the fuel cell, i.e., in the hopper. The hopper occupies a significant portion of the cell volume, effectively reducing the cell volume available for electrochemical activity. It is undesirable to put the hopper zone within the cell frame because the cell frame is an expensive part of the fuel cell system.
Thus, what is needed is an electrochemical power source that maximizes the electroactive portion of each cell by having the electroactive zone occupy substantially all of the available volume of each cell. The cell design should cause minimal shunt currents between cells, and allow the cell to be constantly maintained in a full condition with fuel particles to maximize electrochemical activity. The cell should not require complex filling procedures or long filling times.